I want to make bootable USB drive with Debian and make it bootable on both BIOS and UEFI systems. I've found only separate instructions for BIOS and UEFI systems. Please help me to find a solution.
If you're talking about creating a Debian installation image, I can't provide step-by-step instructions for it; however, the general way to create a USB drive that's bootable in both EFI and Linux is:
The Debian installer, of course, has things set up in its own way, so step #2 will involve figuring out how the installer works and "translating" it to work from the USB flash drive.